Maybe one of them actually doing their jobs? That would at least be
relevant. But I guess it wouldn't be "sexy" and thus not sell as much
(though I would at least buy a copy of /that/).

Yes, that kind of calendar would be good but unfortunately many (if
not most) employees would scream at the thought of someone
photographing the screen of their CTO, security expert, software architect,
QA expert etc. when the person is going their actual work.
For hobby projects it is more easily done as NDAs do not usually play
a part there.